I. Endogenous Factors
Endogenous factors include a group of factors found in animal and human body fluids and tissues. These factors appear to be associated with natriuresis, the excretion of sodium in the urine. Natriuresis arises as a result of inhibition of (Na.sup.+ +K.sup.+)ATPase (the biochemical equivalent of the sodium pump) by the endogenous factors. The sodium pump drives the transepithelial resorption of sodium in animal and human kidneys (De Wardener, H.E. et al, Clin. Sci., 63:415-420 (1982)). However, (Na.sup.+ +K.sup.+)ATPase is not limited to kidney tissue. That is, this enzyme can be found in virtually all animal and human cells.
There is extensive literature on the isolation and purification of endogenous factors that inhibit (Na.sup.+ +K.sup.+)ATPase; the following is only a partial list: Gruber, K.A. et al, Nature, 287:743-745 (1980); Cloix, J.F. et al, Biochem. Biophys. Res. Commun., 131:1234-1240 (1985); Kelly, R.A. et al, J. Biol. Chem., 260:11396-11405 (1985); Fishman, M.C., Proc. Natl. Acad. Sci. USA, 76:4661-4663 (1979); Lichstein, D. et al, Biochem. Biophys. Res. Commun., 96:1518-1523 (1980); Akagawa, K., J. Neurochem., 42:775-780 (1984); Jandhyala, B.S. et al, Clin. Sci., 70:103-110 (1986); Carilli, C.T. et al, J. Biol. Chem., 260:1027-1031 (1985); Clarkson, E. et al, Kidney Internatl., 16:710-721 (1979); Kramer, H. et al, Hormonal Regulation of Sodium Excretion, pp. 303-323 (1980); Cloix, J.F., FEBS Lett., 176:223-228 (1984); de The, H. et al, J. Cardiovas. Pharmacol., 6:549-554 (1984); Devynck, M.A. et al, Clin. and Exper. Hyperten. - Theory and Pract., A6:441-453 (1984); and Tamura, M. et al, J. Hyperten., 5:219-255 (1981).
One such factor or family of factors, i.e., endogenous digitalis-like factor or substance (hereinafter "EDLF" or "EDLS"), is believed to be natriuretic by virtue of its ability to inhibit (Na.sup.+ +K.sup.+)ATPase.
EDLF inhibits (Na.sup.+ +K.sup.+)ATPase in at least three different biochemical assay systems (Hamlyn, J.M. et al, J. Biol. Chem., 264:7395-7404 (1989)). However, EDLF does not cross-react with antibodies raised against digoxin. In this regard, and in terms of chromatographic and kinetic properties, EDLF is different from some of the other previously isolated (Na.sup.+ +K.sup.+)ATPase inhibitors (Graves, S.W. Ann. Int. Med., 99:604-608 (1983); Gruber, K.A. et al, Nature, 287:743-745 (1980); Gruber, K.A. et al, Hyperten., 4:348-354 (1982); Vasdev, S. et al, Res. Commun. Chem. Pathol. Pharmacol., 49:387-399 (1985); Kramer, H.J. et al, Renal Physiol., 5:80-90 (1985); Kelly, R.A. et al, J. Biol. Chem., 260:11396-11405 (1985); and Crabos, M. et al, FEBS Lett., 176:223-228 (1984)).
In at least two instances, digitalis-like factors with chromatographic properties similar to EDLF have been described in bovine adrenal tissue (Tamura, M., Biochem., 27:4244-4253 (1988)); and in human urine (Goto, A. et al, Biochem. Biophys, Res. Commun., 152:322-327 (1988)). The estimated molecular weights, as determined by fast atom bombardment mass spectrometry, of these digitalis-like factors are 336 and 343 Daltons, respectively.
A digitalis-like factor has also been detected in human amniotic fluid (U.S. Pat. No. 4,780,314). The molecular weight of this factor is described as being 150-250 Daltons. In addition, this factor is taught to cross-react strongly with digoxin antibodies, is highly protein bound in plasma and is a substrate for catechol-o-methyl transferase (COMT). These properties distinguish this digitalis-like factor from the EDLF described in the present invention, which is of higher molecular weight, does not cross-react with digoxin antibodies (Hamlyn, J.M. et al, J. Biol. Chem., 264:7395-7404 (1989)), is not highly protein bound in plasma and is not a substrate for COMT.